1. Field of the Invention
The present invention relates to a steering control apparatus, and more specifically, it relates to an electric power steering control apparatus which controls an assisting force applied by a motor to a steering system of a vehicle based on at least the steering torque of a steering wheel and the speed of the vehicle when the steering wheel is steered to turn by a driver.
2. Description of the Related Art
There have been developed electric power steering apparatuses in which the speed of a vehicle and the steering torque applied to a steering column or shaft are detected, and a driving current determined according to the vehicle speed and the steering torque thus detected is supplied to a motor which generates an assisting force given to the steering shaft, so that the motor is thereby driven to rotate, thus assisting the force required to steer the vehicle by means of the rotating force of the motor to provide the driver with a pleasant steering sensation or feeling.
In the past, a DC motor with a brush has been used as a motor which gives an assisting force to a steering shaft of a vehicle, but it is impossible to perform field control with the DC motor and hence conventional electric power steering control apparatuses are not equipped with any means for generating a target current value used to carry out field control.
Moreover, there can be considered a steering control apparatus which employs a brushless motor in place of such a DC motor, as a motor for applying an assisting force to the steering shaft. In this case, owing to the absence of any brush within an electric motor, there will take place almost no abnormality or fault in the motor itself, and it also becomes possible to effect field control, which could not been carried out in the past as referred to above.
A brushless motor control apparatus performs a variety of calculations such as the calculation of a dq-axis target current based on an instruction torque, the detection of currents for respective phases (e.g., u phase and v phase) of a motor, the dq conversion of currents, the calculation of current deviations, the calculation of instruction voltage values, dq inversions, and the output of PWM control patterns. The detected respective phase currents are subjected to dq conversion in such a manner that they are controlled in a feedback manner to make their d-axis component and q-axis component equal to a d-axis target current and a q-axis target current, respectively. The d-axis component of each current means a wattless or reactive current, whereas the q-axis component is proportional to the torque of the motor when the motor is a synchronous motor and when the magnitude of the excitation magnetic field is constant. Therefore, the current feedback control for the synchronous motor is generally carried out in such a manner that the d-axis component of the detected current becomes zero and the q-axis component thereof becomes equal to a target value of the output torque.
In cases where a steering control apparatus is installed on a vehicle with a large weight, a large motor output torque is required, and there will be a problem that in cases where the motor characteristic is of the high-torque and low-rotation type, the output torque of the motor rapidly decreases upon rapid steering. As a result, in the case of an electric power steering control apparatus, there arises a problem that the steering operation rapidly becomes heavy upon rapid steering, whereas in the case of a steer-by-wire steering system, there is a problem in that the actual steering angle of the steered wheels does not follow the steering angle of the steering wheel upon rapid steering.
However, where the motor characteristic is of the low-torque and high-speed rotation type, it is necessary to increase the motor current in order to enlarge the motor output torque, and hence a large-sized motor of high power consumption is required. With a steering apparatus which is to be installed in a limited space, however, it becomes important to suppress the power consumption of a motor used therein and reduce the size thereof.
The present invention is intended to obviate the various problems as referred to above, and has for its object to provide a steering control apparatus which is capable of alleviating a decrease in the output torque of a motor during high-speed steering without increasing the size thereof.
Bearing the above object in mind, according to the present invention, there is provided a steering control apparatus including a motor, a motor current instruction value generation section for generating a current instruction value for the motor, and a motor current detection section for detecting a current flowing through the motor, the motor being driven to operate based on at least the current flowing through the motor and the current instruction value, wherein the motor current instruction value generation section includes a correction section for correcting a current instruction value which controls a magnetic field of a field magnet of the motor, the correction section being operable to correct the current instruction value for controlling the magnetic field of the motor field magnet when a steering speed is high,
In a preferred form of the present invention, the steering control apparatus further comprises a motor control section for performing torque control on the motor in accordance with a torque instruction through vector control which is represented by a two-phase rotating magnetic flux coordinate system having a direction of a field current oriented in a d-axis direction and a direction perpendicular to the d-axis oriented in a q-axis direction, wherein the correction section corrects a d-axis current instruction value in such a manner that a d-axis current is controlled to such a predetermined value as to weaken the magnetic field of the motor field magnet when a deviation between the q-axis current instruction value and a q-axis current detection value becomes not less than a predetermined value.
In another preferred form of the present invention, the correction section increases a negative d-axis current instruction value for weakening the magnetic field of the motor field magnet when the q-axis current deviation is not less than a first predetermined value, and decreases the negative d-axis current instruction value for weakening the magnetic field of the motor field magnet when the q-axis current deviation is not greater than a second predetermined value.
In a further preferred form of the present invention, the d-axis current instruction value is limited within a preset range.
In a still further preferred form of the present invention, the steering control apparatus further comprises a stator phase current instruction value generation section for generating stator respective phase current instruction values from the q-axis current instruction value, wherein the current instruction value for controlling the magnetic field of the motor field magnet is corrected based on a deviation between at least one of the stator phase current instruction values and an actual corresponding stator phase current value in place of the q-axis current deviation.
In a yet further preferred form of the present invention, the steering control apparatus further comprises a reference steering torque generation section for generating a reference steering torque which is used to determine whether the magnetic field of the motor field magnet is to be weakened, wherein when the steering torque becomes not less than the reference steering torque, the correction section corrects the current instruction value to such a prescribed value as to weaken the magnetic field of the motor field magnet.
In a further preferred form of the present invention, the reference steering torque generation section generates the reference steering torque as a function of at least a vehicle speed.
In a further preferred form of the present invention, the steering control apparatus further comprises a steering speed detection section for detecting a steering speed of a steering wheel, wherein the correction of the current instruction value for controlling the magnetic field of the motor field magnet is effected such that when the steering speed of the steering wheel becomes not less than a predetermined value, the current instruction value is corrected to such a prescribed value as to weaken the magnetic field of the motor field magnet.
In a further preferred form of the present invention, the steering control apparatus further comprises: a motor control section for performing torque control on the motor in accordance with a torque instruction through vector control which is represented by a two-phase rotating magnetic flux coordinate system having a direction of a field current oriented in a d-axis direction and a direction perpendicular to the d-axis direction oriented in a q-axis direction; a speed detection section for detecting a rotational speed of the motor; and a voltage limitation value generation section for generating a voltage limitation value to a voltage applied to the motor; wherein the correction section determines a d-axis current value for setting a working point on a voltage limitation circle through calculations based on at least the rotational speed of the motor, a q-axis current instruction value, a stator winding resistance, a stator winding reactance and a motor counter electromotive voltage constant, and effects correction in such a manner that when the d-axis current value determined through calculations is a current value which weakens the magnetic field of the motor field magnet more than a d-axis current instruction value does, the d-axis current value determined through calculations becomes equal to the d-axis current instruction value.
In a further preferred form of the present invention, the steering control apparatus further comprises a power supply voltage detection section for detecting a power supply voltage, wherein the voltage limitation value generation section generates, as a voltage limitation value, a value obtained by multiplying the power supply voltage by a predetermined coefficient.
In a further preferred form of the present invention, the motor comprises a field winding type motor, and the apparatus further comprises a motor control section for performing torque control on the motor in accordance with a torque instruction in such a manner that a field winding current instruction value is corrected when a deviation between an armature current instruction value and an armature current detection value becomes not less than a predetermined value.
In a further preferred form of the present invention, the field winding current instruction value is corrected in such a manner that it is decreased when the armature current deviation is not less than a first predetermined value, and it is increased when the armature current deviation is not greater than a second predetermined value.
In a further preferred form of the present invention, the field winding current instruction value is limited to a preset minimum value.
In a further preferred form of the present invention, the correction section corrects the current instruction value with a correction value which is determined through calculations.
In a further preferred form of the present invention, the correction section corrects the current instruction value with a correction value which is obtained by referring to a table prepared in advance.
In a further preferred form of the present invention, the d-axis current instruction value is limited to a value which is obtained by vector subtracting a q-axis current value from a preset maximum current vector value.
In a further preferred form of the present invention, an integrated value of the deviation is used in place of the deviation.
In a further preferred form of the present invention, the correction section corrects the current instruction value for controlling the magnetic field of the motor field magnet only when a vehicle speed is not less than a predetermined value.
In a further preferred form of the present invention, the correction section corrects the current instruction value for controlling the magnetic field of the motor field magnet only when a steering torque is not less than a predetermined value.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings.